1. Field of the Invention
The present invention relates to an opto-electric conversion semiconductor device, and more particularly to, an opto-electric conversion semiconductor device having a superior opto-electric conversion characteristic over a wide frequency range for optical communication.
2. Background Art
An improvement in the performance of an opto-electric conversion semiconductor device for interconnecting a terminal electric line and an optical-fiber network plays an important role in attaining prevalence of a public communication network using optical fibers.
High-speed modulation for responding to an increase in the volume of transmitted information is indispensable for improving the performance of a semiconductor laser device or a photodiode that serves as an opto-electric conversion device. Further, an important requirement is that the semiconductor device or the photodiode has a good opto-electric conversion characteristic over a wide frequency range.
In order to effect high-speed modulation of a semiconductor laser device, an external modulation method is employed. According to this method, in order to enable long-distance transmission of a signal while reducing variations in the wavelength of the signal, which would occur during modulation, a signal is modulated by being passed through an optical modulator which is capable of controlling the amount of light passing therethrough through on-off operation in accordance with an optical signal, by means of causing a semiconductor laser to oscillate at a given intensity.
Control of the amount of light passing through the optical modulator through on-off operation, which is employed for the external modulation method, is achieved by means of the Franz-Keldysh effect, which arises when a reverse electric field is applied to an absorbing layer of an optical modulator, or by means of the quantum confinement Stark effect.
In the optical modulator, the absorption of a laser beam is changed according to a voltage to be applied to the optical modulator. Hence, a modulated signal voltage is applied to a high-frequency electric circuit connected to the optical modulator, and the laser beam to be output from an exit-end face of the optical modulator is subjected to intensity modulation in accordance with the signal voltage.
In a case where such an optical modulator is connected to a high-frequency electric circuit for enabling propagation of a modulated signal voltage, impedance matching must be ensured over the frequency range from d.c. to a modulation frequency, because the modulated signal voltage usually assumes a rectangular waveform. To this end, the amount of reflection attenuation, which would arise at a modulation frequency, must be increased, and the optical modulator must have a cut-off frequency sufficiently exceeding the modulation frequency.
The external modulation method encounters difficulty in establishing optical coupling between an optical modulator and a semiconductor laser and requires a large number of components, which adds to cost. In order to eliminate such a difficulty, there has been developed an optical modulator integrated semiconductor laser device which is formed by monolithic integration of a semiconductor laser and an optical modulator.
As in the case where the optical modulator is connected to a high-frequency electric circuit, the optical modulator integrated semiconductor laser device requires impedance matching over a wide frequency range from d.c. to a modulation frequency.
The foregoing description applies to a device for converting an electric signal into an optical signal. However, a photodiode for converting an optical signal into an electric signal also requires impedance matching over a wide frequency range from d.c. to a modulation frequency.
FIG. 25 is a plan view showing a commonly-known optical modulator described in, for example, Japanese Patent Laid-Open No. 38373/1999.
In FIG. 25, reference numeral 200 designates an optical modulator; 202 designates an optical modulation element; 204 designates a high-frequency electric circuit; 206 designates a high-frequency substrate; 208 designates a transmission line; 210 designates a matching circuit; 212 designates an open stub; and 214 designates a metal wire.
Arrow 216 designates incident light which corresponds to continuous light and originates from a laser; arrow 218 designates signal light modulated by the optical modulation element; and arrow 220 designates an electric signal which is applied to the optical modulation element 202 in the form of variations in a voltage by way of the high-frequency electric circuit 204.
The optical modulator 200 comprises the high-frequency electric circuit 204 having the matching circuit 210 formed from the open stub 212; the optical modulation element 202 provided at the tip end of the high-frequency electric circuit 204; and the metal wire 214 for connecting the optical modulation element 202 with the high-frequency electric circuit 204.
The commonly-known optical modulator 200 having the foregoing configuration operates as follows:
Upon efficient receipt of the incident light 216 by way of a coupling optical system (not shown), the optical modulation element 202 modulates the intensity of the incident light 216 in accordance with a variation in the voltage of the electric signal 220 by way of the Matching circuit 210 of the high-frequency electric circuit 210 and the metal wire 214, thereby emitting the signal light 218.
In this case, before entering the metal wire 214, the electric signal 220 is subjected to impedance matching performed by the matching circuit 210 formed from the open stub 212.
The open stub 212 corresponds to a capacitive matching circuit, and impedance matching is effected by means of only the matching circuit 210 formed from the open stub 212, and hence impedance matching can be achieved in the vicinity of only a certain frequency. Accordingly, the open stub 212 can improve the modulation characteristic of the optical modulator 200 within a narrow range but encounters difficulty in improving the modulation characteristic of the optical modulator 200 over a wide range.
Impedance matching is commonly achieved by use of only a terminating resistor. Use of only a terminating resistor may lead to matching for d.c. However, because of a parasitic capacitance of the optical modulation element 202 or the inductance of the metal wire used for electrical connection, an impedance mismatch arises in, particularly, a high frequency range, thereby rendering difficult attainment of a match over a wide frequency range.
The present invention has been conceived to solve the above-described drawbacks and is aimed at providing an opto-electric conversion semiconductor device which comprises a resistive matching circuit connected to a capacitive matching circuit by way of an opto-electric conversion semiconductor element and achieves impedance matching over a wide frequency range.
For reference, Japanese Patent Laid-Open No. 75003/1998 describes a semiconductor laser module for directly receiving a microwave frequency signal as a modulated signal, in which an impedance matching circuit unit utilizing capacitance is interposed between a laser diode chip and a signal input circuit.
Further, Japanese Patent Laid-Open No. 221509/1995 describes an invention which uses, as a terminating resistor formed from a chip resistor, a capacitive matching circuit for canceling the inductance component of a chip resistor.
According to one aspect of the present invention, an opto-electric conversion semiconductor device comprises a semiconductor element for opto-electric conversion of a signal which has an signal input electrode. A high-frequency electric signal circuit is provided which has one end positioned in proximity with the semiconductor element. The high-frequency electric signal circuit has a connection point in the one end at a location closest to the signal input electrode of the semiconductor element, and the connection point is connected to the signal input electrode of the semiconductor element by way of a conductor. A resistive matching circuit is connected to the signal input electrode at one end and to ground at the other end. A capacitive matching circuit is connected to the connection point of the high-frequency electric signal circuit. The capacitive matching circuit has an adjusted impedance such that an impedance at the connection point towards the semiconductor element is same with a normalized impedance of the resistive matching circuit.
According to another aspect, an opto-electric conversion semiconductor device comprises a semiconductor element for opto-electric conversion of a signal which has a signal input electrode. A high-frequency electric signal circuit is provided which has one end positioned in proximity with the semiconductor element. A resistive matching circuit is connected to the electric signal terminal by way of a conductor at one end and to ground at the other end. A resistance matching correction circuit is connected between the signal input electrode and the one end of the high-frequency electric signal circuit by way of a conductor. A capacitive matching circuit is connected to the one end of the high-frequency electric signal circuit. The capacitive matching circuit, has an adjusted impedance such that an impedance at the one end towards the semiconductor element is same with a normalized impedance of the resistive matching circuit in the vicinity of a desired frequency range.
In another aspect, in the opto-electric conversion semiconductor device, the semiconductor element may comprise either of an optical modulator element, an integrated semiconductor laser element equipped with an optical modulator, a photo sensitive element, and a flip-chip type semiconductor element.
Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings.